Background of the Invention When aluminium is melted e. g. for manufacture of extrusions ingots and billets, because of the influence of oxygen from environmental air on the aluminium and the existence of impurities, particularly oxides, nitrides and carbides, in the molten aluminium, a layer of sludge, also known as dross rises to the surface of the molten aluminium. This layer of dross has to be removed from the molten aluminium before the molten aluminium can be cast. This is done by the use of a suitable ladle. In a rather crude process, known as skimming. the ladle is dragged across the top of the molten aluminium and the dross is scraped into a suitable receptacle. During the skimming process, as well as removing sludge including oxides and other impurities, pure aluminium is also removed. The quantity of pure aluminium removed depends on the depth to which the ladle is inserted in the aluminium to ensure removal of all the dross and to a large extent depends on the skill of the furnace worker handling the ladle. However, typically 30 to 60% of the mixture/dross, by weight is aluminium.

The term dross, as used herein, refers to the impurities such as oxides which float to the surface of the molten aluminium, but the term is also used in the art to refer to the mixture of aluminium and the impurities.

Because of the amount of aluminium in the mixture, it is obviously desirable to remove as much aluminium from the dross/aluminium mixture as possible. Almost all recycling is done by a process known as rotary salt furnace processing. In that process, the dross containing pure aluminium is first allowed to cool. The longer the aluminium spends hot, the more oxidation occurs and less aluminium is recovered in the recycling process, so often cooling is encouraged and accelerated. In some cases some initial

separation of aluminium from the mixture is first carried out by one of two rather inefficient devices know as drain pans and dross presses, respectively.

In the former the mixture is allowed to sit while molten and some of the aluminium will sink to, and agglomerate in, the bottom of the pan. In the latter, the mixture is compressed and the aluminium droplets tend to stick together. These processes are inefficient and have to be followed by rotary salt processing or other methods of external dross processing. Because the mixture is kept hot longer for the drain pan or dross press process, the recovery rate in the subsequent rotary salt process is reduced, so drain pans and dross presses are generally not commercially viable, and not often used.

Recycling is not generally done at the furnace, but is usually is done by specialist metal-recycling companies. In the rotary salt recycling process, the dross is heated and remelted and various salts and fluxes are added in order to separate the aluminium from the oxides and other impurities. While the process is highly efficient in terms of the quantity of aluminium removed from the dross, removing approximately 85% of the available aluminium, the waste product from the recycling process, ie the mixture of salts and the oxides, is unpleasant, very environmentally unfriendly, and difficult to dispose of safely. Further, the process requires the transporting of the dross to the recycler in lorries or the like which is also undesirable from an environmental point of view, and inefficient in terms of fuel. Also, the dross has to be remelted in order to extract the aluminium in the recycling process which requires a substantial amount of energy. There are some smelting plants which have their own rotary salt recycling furnace, however, the process of cooling and transporting the cooled mixture to the furnace remains the same, although savings are made in total transport costs.

Proposals have been made for separating aluminium from dross in the past. GB 1533696 and US 3689049 disclose two different device for separating aluminium from dross. Neither device has had any commercial success, perhaps because they are over-complicated and too unreliable for the extreme environment in which they have to operate.

The present inventor has also invented an apparatus and process, disclosed in AU 56260/98, that provides a simpler and more cost effective method of recycling aluminium from dross. The present invention is directed to improvements in the apparatus earlier developed by the inventor.

Summary of the Invention According to a first aspect, the present invention is an apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross including: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross, the table being adapted to vibrate so as to vibrate the crucible; a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and means for relatively lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture; and a screening means able to substantially surround the crucible when it is supported by the table, the screening means having at least a portion thereof movable between at least a first retracted position to allow access to the crucible and at least a second position where it prevents access to the crucible.

The screening means can be removably mounted to the frame. In this embodiment, the movable portion may comprise a door that is retractable to allow access to the table of the apparatus. The door can be adapted to move upwardly, sidewardly or downwardly to provide a portal for access to the table. The door can be slidably movable relative to the remainder of the screening means. In another embodiment, the door can be pivotally mounted to the wall. When the door is fully retracted, the portal must be of sufficient size to allow the crucible to pass through the portal.

In another embodiment, the entire screening means can be movable between the first retracted position and the second position. In this embodimelt, the screening means can extend downwardly from the lowering means for the paddle means and is so movable relative to the table and crucible in concert with the operation of the lowering means.

The screening means is preferably adapted to prevent or at least ameliorate the escape of heat from the vicinity of the crucible. By preventing the escape of heat, the screening means serves to protect workers working near the apparatus. It also serves to slow the rate of cooling of the crucible and its contents which is desirable. The screening means can also preferably be adapted to prevent or at least substantially prevent the escape of dust and

flame from the vicinity of the crucible. The screening means can also be adapted to safeguard workers working in the vicinity of the apparatus from explosions of aluminium and dross from the crucible.

The screening means is preferably formed from a multi-layer structure, comprising at least an inner layer and an outer layer. The inner and outer layer can be fabricated from a suitable metal or other material, including refractory or cementitious materials.

The paddle or stirring means can comprise a plurality of blades mounted on a shaft supported by a frame, the shaft being adapted to be raised or lowered so that the blades may be raised when a new crucible is placed on the table located underneath the blades. In one embodiment, in use, the shaft is positioned at or near the centre of the crucible and is rotatable by a rotating means about its longitudinal axis. In a further embodiment, the shaft, in use, is radially offset from the centre of the crucible and is in addition to rotating about its longitudinal axis is also rotated by the rotating means about the centre of the crucible thereby providing more thorough stirring of the crucible's contents. In a still further embodiment, a gear is provided such that the shaft undergoes a planetary motion as it is rotated by the rotating means about the centre of the crucible.

This further rotation of the blades leads to a even more thorough stirring of the crucible's contents.

The crucible may be made of a refractory material having a metallic outer skin spaced from the refractory material by a layer of insulation. The crucible can be provided with a covering means that at least partially covers the upper opening of the crucible. The cover can be retractable or otherwise removable from the crucible when the paddle or stirring means is relatively lowered into the crucible. If desired, the cover can be adapted to surround the shaft of the paddle or stirring means while the paddle or stirring means is stirring the crucible's contents.

The table may be mounted on springs to assist in vibrating the crucible. Vibration of the table and crucible may be achieved by the mounting of at least one ultrasonic transducer, electric vibrator, mechanical vibrator or a combination of one or more of these transducers to the crucible and/or table. The table can be vibrated such that it oscillates in a horizontal axis, a vertical axis or in both axes.

The frame may consist of two or more upright pillars. A cross-beam may be supported by the frame and that cross-beam may carry the rotating means. The shaft for the blades may depend down from the cross-beam with the blades mounted on the lower end of that shaft.

The apparatus can include one or more exhaust pipes that allow gaseous product from the process performed by the apparatus to be vented to atmosphere outside the screening means. The exhaust pipes can incorporate bagging systems that collect the exhaust gaseous product for later disposal.

The bagging systems can be adapted to also or instead extract dust from the gaseous product. The exhaust systems can also include filtering or scrubbing means that clean or substantially clean the gaseous product from the process of harmful or potentially harmful constituents.

The apparatus can include a weighing means, such as an electronic or mechanical scale, that allows measurement of the weight of the crucible and its contents while the crucible is sitting on the table. The apparatus can also include a control means. The control means is preferably adapted to allow an operator to operate all features of the apparatus from a central location. In a further preferred embodiment, the control means can be automated such that the apparatus runs normally without operator input.

Thus in a further aspect of the present invention there is provided an apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross including: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross ; a weighing means, such as an electronic or mechanical scale for measuring the weight of the crucible and its contents while the crucible is sitting on the table; a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and means for relatively lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture; a sensor means for sensing the pressure on the paddles during stirring; and a control means arranged to determine the optimum stirring time from the measurements of pressure on the paddles and/or the weight of the crucible and contents.

The control means may also control the sequence and rate of the vibrations.

While the size of the crucible containing the dross is not important, typically the crucible should be large enough to carry between 800 kilograms to 1.5 tonne of dross. The process takes approximately 10 minutes and removes up to 95% of the aluminium in the dross, hence the present invention can separate approximately 1000 kilograms of aluminium from dross every 4-10 minutes.

Typically, the crucible will include plugholes which are plugged prior to carrying out the separation process with for example a cone or a sand and clay solution, and are unplugged to drain the crucible into a draining pan.

The draining pan preferably has a cover, having an inlet port, that encloses the draining pan. In use, aluminium being drained from the crucible at the completion of the process cycle passes through the inlet port and into the further draining pan. As there is risk of an explosion if the molten aluminium comes into contact with moisture in the further draining pan, the cover preferably contains any such explosion should it occur thereby ensuring the safety of workers close thereby.

In an alternative embodiment, the apparatus can include an additional drain pan adapted to be positioned directly beneath the crucible which is attachable to the crucible and so movable, if desired, with the crucible when the crucible is moved in and out of the apparatus. Where the crucible has a drain pan, the crucible would preferably have one or more drain holes in or adjacent its bottom surface and in fluid communication with the drain pan so that aluminium can drain into the drain pan. The provision of a drain pan is advantageous as it allows for collection of aluminium which moves to the bottom of the crucible to be readily retrieved from the crucible. It has particular application in those instances where the aluminium content of the aluminium/dross mixture is relatively high, ie about 65-85% aluminium. If the aluminium cannot drain in these instances, it is possible that previously molten aluminium in the mixture closer to the top of the crucible will begin to solidify before completion of the stirring and vibrating process according to the present invention. By lowering the level of the mixture during the process, using the drain pan, it lessens the likelihood of solidification in the crucible before completion of the process.

The one or more drain holes that are in fluid communication with the drain pan can be sealable by a sealing means. Each sealing means can be mounted to the crucible at all times and be moved in or out of sealing engagement with its respective drain hole as desired.

It will be appreciated that the apparatus could be adapted to receive more than one crucible at a time. For example, the apparatus could comprise two or more tables and stirring means to allow the contents of two or more crucibles to be stirred simultaneously. In another example, the apparatus could comprise two or more tables but only one stirring means. In this embodiment, the contents of each crucible would be stirred sequentially. As such, the option would be open for the contents of one crucible to be stirred and vibrated while another crucible is being filled with dross mixture and positioned in the apparatus. When the process cycle was complete on the first crucible, the stirring means would be relatively raised, moved and then relatively lowered into the next crucible. While this is occurring, the recovered aluminium can be drained from the first crucible and it can be removed from the apparatus, filled with the next quantity of dross mixture to be processed and then returned to the apparatus ready for the stirring means to again be moved back and lowered again into this crucible. This sequential mode of operation is particularly advantageous as it minimises the down time where the stirring means is not in operation.

In a further aspect, the present invention is an apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross including: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross; at least one ultrasonic transducer, electrical vibrator or mechanical vibrator, adapted to vibrate the table either directly or indirectly; a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and means for relatively lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture.

According to a further aspect, the present invention comprises an insulated crucible able to contain a mixture of molten aluminium and aluminium dross, and a drain pan that is positioned immediately below or adjacent and is removably attachable to the crucible, the drain pan being

adapted to receive molten aluminium that flows through at least one sealable drain hole in the crucible.

The one or more drain holes in the crucible are preferably sealable by a sealing means that is mounted on the crucible and is movable in or out of sealing engagement with its respective drain hole as desired.

In yet a further aspect, the invention involves a method for separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of: removing the mixture from a furnace containing molten aluminium; transferring the hot dross to an insulated crucible; transferring the crucible to a table means; inserting a paddle means into the mixture in the crucible; relatively lowering a screening means such that it substantially surrounds the crucible; stirring the mixture with the paddle means and simultaneously vibrating the table and crucible to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce, such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and removing aluminium from the lower part of the crucible.

In still yet a further aspect, the invention involves a method for separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of: removing the mixture from a furnace containing molten aluminium; transferring the hot dross to an insulated crucible; transferring the crucible to a table means; inserting a paddle means into the mixture in the crucible; stirring the mixture with the paddle means and simultaneously vibrating the table and crucible using at least one ultrasonic transducer, electric vibrator, mechanical vibrator or a combination of one, two or all to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce, such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and removing aluminium from the lower part of the crucible.

The pure aluminium can then be either cast as an ingot or recycled straight back into the furnace containing molten aluminium or drained into a crucible.

The process is carried out without substantially cooling the dross, preferably at a temperature of about 750°C.

In a yet further aspect the invention provides an apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross including: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross; at least one heating means such as a gas burner of the like adapted to either preheat the paddles or heat the contents of the crucible or heat both. a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and means for relatively lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture.

The gas burners of the like can be used to maintain the heat in the contents of the crucible during the stirring process. In contrast with prior art devices such as dross presses maintaining heat during the separation process has been found to be desirable.

Brief Description of the Drawings By way of example only, preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic and simplified illustration of the present invention; Figure 2 is a detailed drawing of apparatus embodying the present invention; Figure 3 is a detailed drawing of the apparatus depicted in Fig. 2 but with additional features; Figure 4 is a cross-sectional view of another embodiment of a crucible for use in the present apparatus; Figure 5 is another view of the crucible of Figure 4 ; Figure 6 is a simplified plan view of the crucible of Figure 4; Figure 7 is a side view of a variation on the separation apparatus shown in Figure 2; and Figure 8 is a drawing of a bucket with lid.

Detailed Description of the Preferred Embodiment Referring the to the drawings, Figure 1 shows a forklift truck 10 transporting a crucible containing dross at a temperature about 750°C to an apparatus, generally indicated at 14, for removing aluminium from the dross.

The crucible is placed on a table 16 of the apparatus. Blades 18 are lowered into the molten dross, the table 16 is vibrated in a vertical direction, as illustrated by the arrows AA, the blades 18 rotate in the molten dross, and by virtue of that stirring and vibration, the aluminium droplets and particles in the dross coalesce to form larger droplets and gradually sink to the base of the crucible. The aluminium can then be drained out into a bucket 20 and either transferred directly back to the furnace or used to produce an aluminium ingot 22.

Figure 2 shows a more detailed drawing of the apparatus of the present invention. The device includes a frame comprised of a number of upright pillars or rails 50, a base 52 and cross beam 54. In plan view the frame includes four pillars located on the corners of a square. A table 54 is mounted on squat pillars 56 which rise up from the base 52. A number of springs 58 also extend between the base 52 and the table 54. On top of the table a crucible 59 is located. The crucible can be generally square or circular in plan view and has sides which taper outwardly. The inside of the base of the crucible is shaped like a pyramid so that no sharp corners exist, so that the dross can be stirred properly and so that the crucible can be cleaned easily. A pipe or channel 60 is formed in the base of the crucible which, when unblocked, allows molten metal to flow out from the crucible.

The crucible is made of a refractory material. having an outer skin of metal and an insulating layer disposed between the skin and the refractory material. Although refractory material has insulating properties, the additional layer further helps to prevent the dross cooling as it is transported to the table. A shaft 62 depends from the cross-beam 54 and on the lower end of the shaft there are a series of blades or paddles 64 for stirring the contents of the crucible. The shaft is movable relative to the cross-beam in the vertical direction to raise and lower the blades 64. A motor is also provided on the shaft to rotate the shaft in the direction B to thus stir the contents of the crucible by means of the blades 64. While not depicted, it can be envisaged that each of the blades 64 could be smaller than depicted, with the shaft 62 adapted to rotate both about its own axis and that of the

crucible 59. Other stirring arrangements for the blades 64 such as described above could also be readily envisaged.

In use, dross is taken from the furnace and placed straight into a crucible 59 and the crucible moved by a forklift or the like and placed directly on the table 54. Once the crucible is in place on the table, the blades are lowered into the dross. The table on which the crucible sits is then vibrated in the vertical direction, at a rate of between 500 to 5,000 or more vibrations per minute. In the embodiment depicted in Fig. 2, the vibration of the table is achieved through use of electrical vibrators 70 mounted under the table 54. In other embodiments, the vibration may be achieved through use of one or more ultrasonic transducers, mechanical vibrators, or a combination of such vibrators mounted to the table 54. It is also possible to mount the electrical vibrators on the sides of the crucible or frame to provide horizontal vibration in addition to, or instead of, the vertical vibrations.

In normal operation, the blades 64 turn at a rate of between 3 to 40 rpm. The vibrations break the oxide layer surrounding the droplets of aluminium in the dross and allow the metal droplets to coalesce with the result that the droplets then become larger in volume and tend to sink to the bottom of the crucible where they can flow through the hole 60 into another bucket or drain pan. As shown in Figure 9 the other bucket or drain pan 20a can have a cover 80 having an inlet orifice 82 to allow aluminium to drain into the bucket in a safe manner. The rate of vibration can change during the process and tends to start more slowly and then increase later for best results.

High vibrational rates are used to bond very small drops of liquid metal together and by utilising higher vibrational speeds a larger percentage of pure metal can be recovered. As discussed above, generally, the content of pure metal in dross from aluminium furnaces is usuallv between 30 to 60%, and once the process described above has been carried out on the dross the content of the remaining aluminium in the dross tends to be in the range of 3 to 5%. Both stirring and vibrating is needed.

The specific embodiment described above refers to vibrating the crucible in a generally vertical axis, and generally horizontal stirring, stirring and vibration could take place in other axes and by other methods. The important feature of the invention is that sufficient agitation, vibration, or stirring occurs at rates which cause the aluminium droplets to coalesce.

In a further embodiment of the invention as depicted in Figure 3, walls 71 are shown mounted to the frame 50 of the apparatus 14. The walls are a multi-layer structure comprising a metallic inner layer and a metallic outer layer. The walls 71 serve to prevent heat, dust, flame and explosion from escaping the vicinity of the crucible and endangering any workers working nearby. The walls 71 also serve to lessen the rate of temperature drop of the crucible and its contents during operation of the apparatus. Disposed on one face of the apparatus is an opening 72 provided by a door that can be raised or lowered as desired. In the depicted embodiment, the door is not visible as it has been retracted behind the upper wall 71. The opening is of sufficient size to allow the forklift to insert the crucible into the apparatus. In Fig. 3, the device is depicted just after the crucible has been placed on the table 54 and with the blades 64 lowered into the crucible ready to stir the contents of the crucible. In normal operation, it would be anticipated that the door would be left open for a short as time as possible. Accordingly, in normal operation, it would be expected that the opening 72 would be closed by the door immediately after the crucible is placed on the table or as the blades 64 are lowered into the crucible.

An alternative crucible to that depicted in Figures 1 to 3 but which could be used in the apparatus and process described herein is depicted generally as 75 in Figures 4 to 6. In this embodiment, the crucible 75 has a drain pan 76 positioned immediately under and removably attached to the lower surface of the crucible 75. Located in the lower surface of the crucible are a plurality of drain holes 77 through which the aluminium can drain into the drain pan 76 in use. The drain pan 76 is particularly advantageous for those smelters having a high proportion of metal to dross in the dross mixture. In such cases, the higher proportion of aluminium can lead to the aluminium solidifying near the top of the mixture before the cycle of stirring and vibration is complete. By providing the drain holes 77, the aluminium falling to the bottom of the crucible 75 can drain from the crucible 75 thereby lowering the level of dross mixture in the crucible and so reduce the likelihood of the aluminium in the remaining mixture solidifying before completion of the cycle.

Figure 7 shows a variant of the apparatus shown in Figures 2 and 3 having a number of additional features. First a hood 83 is provided and exhaust pipe 84 are also provided to allow safe extraction of dust and/or

gaseous product from the process. A bagging system 86 for the capture of dust passing through the exhaust pipe is also provided so that gases leaving the exit 88 of the pipe are clean.

Figure 7 also illustrates the provision of two gas burners 88 and 90.

The gas burner 88 is used to preheat the paddles and burner 90 can be used to heat the surface of the contents of the crucible to assist in preventing solidification of the aluminium.

A further additional feature is the provision of a weighing means such as electronic scales adapted to weigh the crucible and contents and transmit that information to a computer control unit 94. The control unit also receives inputs from sensors 96 associated with the paddle drive means which measure the force required to turn the paddles. From this information the control means can determine the optimal stirring time and feed control signals to a control box 94 which controls the rate and duration of stirring.

The control means can also be used to control the sequence and rate of vibrations.

Although the above process has been described as being used with aluminium, it could be possible to employ the process in other processes having metals of similar properties to aluminium.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.